1. Field of the Invention
The present application is directed toward a coating for an exterior surface of a telescope lens, especially a riflescope, a spotting scope, or a binocular to reduce the likelihood of fogging or distortion due to the collection of moisture on the lens without significantly reducing light transmission through the lens in the visible range. More particularly, the present invention is directed toward such coatings that are durable. Still more particularly, such coatings may include two layers with the first layer being an anti-reflective (AR) coating which has either a single layer or multiple layers such that light transmission in the visible range is not significantly reduced and the second coating is a durable hydrophobic coating over the AR coating. Still more particularly, the present invention provides a coating which is not easily scratched or worn away undergoing standard durability and abrasion testing.
2. Description of the Prior Art
Scopes used by sportsmen, military and the like, have improved significantly over the years. However, much of this improvement is lost on cold days or rainy days due to collection of moisture or fogging on the lens which then significantly distorts the image. For example, a hunter on a cold day will often bring the gun to his face to aim through a scope at a target, and in doing so often breathes on the scope""s lens. The hunter""s breath then fogs the lens. On days when it is raining or drizzling, the moisture from the rain can likewise collect on the lens and distort the image. While the interior face of the lens can be protected against the elements by placement of a moisture free gas within the interior of the scope, the exterior face of the lens is invariably exposed to the elements and incurs the fogging and moisture collection noted above.
Various treatments have been previously utilized that apply hydrophobic materials to various articles, such as sunglasses and automobile windshields, to produce beading of water droplets/or the quick sloughing of moisture, so that the moisture is not retained and spread unevenly on the article due to surface tension so as to produce distortion. However, such treatments reduce the transmissions of light through the article. Typically this reduction in transmission occurs even if the treatment is essentially clear or see through, since the index of refraction of the treatment is different in comparison to the index of refraction of the article causing some amount of reflection to occur. The light that is reflected is then not seen by the viewer which reduces the quality of the image. In articles such as windshields and sunglasses, the amount of reduction in light transmission is not critical, and therefore, a loss of a small amount is not considered to be extremely important.
However, in riflescopes and other telescopic devices, it is extremely important to maintain light transmission in the visible range near 100 percent of incident light in order to ensure that the best possible quality image is seen through the scope. Conventional treatments for rendering articles hydrophobic reduce the transmission of light in certain visible ranges sufficiently to make use of such treatments unacceptable.
Anti-reflective treatments are used on various materials to decrease reflection and increase transmission of light, especially in certain wavelengths. Anti-reflective treatments have been previously used in the prior art for various types of devices, including lenses. Anti-reflective treatments have been especially used where lenses with multiple components are joined together in a side-by-side (or layered) relationship. However, hydrophobic polymers that naturally reduce transmission of light do not adhere to most of the components of anti-reflective treatments that increase transmission of light. Even if the hydrophobic polymer will adhere to an anti-reflective, it has been found through testing that the hydrophobic polymer can often easily be removed by rubbing or just general wear over time.
Consequently, it is desirable to provide an overall coating for an exterior face of a riflescope lens or the like, which includes an externally located hydrophobic polymer that transmits light in the visible range and causes quick beading and sloughing of moisture from the lens. Furthermore, the hydrophobic polymer requires an anti-reflective component that is designed to allow almost one hundred percent of visible light at selected wavelengths to be transmitted through the lens and the hydrophobic polymer. Still furthermore, it is important that the hydrophobic coating be strongly adhered to the lens and not easily removed by rubbing or wear over time. Additionally, what is needed is a hydrophobic lens coating having a certain hardness or durability which provides a certain degree of protection for the lens or any AR layers located on the lens such that the coating is not easily scratched. In this respect, it is desirable that the hydrophobic lens coating be of sufficient hardness and durability to undergo conventional testing without significant damage to the hydrophobic coating, any AR coating, or the lens itself.
The present invention overcomes the problems inherent in the prior art and provides a distinct advance in the state of the art. A coating is provided for lenses to render the exterior face or surface of the lens hydrophobic to reduce the likelihood of fogging and to reduce the likelihood of distortion of images passing through the lens by collection of water thereon. The coating comprises two layers; the first layer being an anti-reflective treatment and the second layer being a light transmitting hydrophobic polymer treatment.
The anti-reflective layer includes multiple sublayers, the material of construction of which and the thickness of which are chosen to provide the best transmission of selected wavelengths through the lens in conjunction with the particular hydrophobic polymer utilized. The number of sublayers in the anti-reflective layer may vary in accordance with design techniques and typically range from three to seven in number. Preferably, the anti-reflective layer has four sublayers.
The outer layer of the anti-reflective layer, that is the layer opposite the lens and adjacent the hydrophobic layer is constructed of silicon dioxide. The anti-reflective layer is applied in ways well known in the art. However, in order to make the hydrophobic polymer layer wear resistant, it is best to heat the anti-reflective layer above ambient temperature. Preferably the anti-reflective layer is heated to a temperature in the range of 250-300xc2x0 centigrade provided that the lens is constructed of glass that can withstand such temperatures. In alternative methods, the anti-reflective coating is applied to a lens using vapor deposition or sputtering.
Alternatively, the outer layer of the anti-reflective layer can be formed of any conventional AR coating and it is desirable to select each AR layer based upon the desired AR characteristics of the final product. Of course, this is also true for single-layer AR coatings.
While the anti-reflective layers may be constructed of numerous types of materials, one particular preferred embodiment includes 70 nanometers of aluminum oxide (Al2O3), 70 nanometers of ZrO3, 225 nanometers of MgF2 and 140 nanometers of SiO2 where it is desired that the wavelength of visible light at 550 nanometers be most clearly and completely transmitted through the lens.
One preferred method of applying this coating (including both an AR layer and a hydrophobic coating) generally includes the steps of applying an AR coating to a lens under vacuum in a first chamber and then having the hydrophobic coating applied to the AR-coated lens applied using vapor deposition in a second vacuum chamber. Another preferred method applies the AR coating to a lens using sputtering with the final layer being sputtered on being the hydrophobic coating. This process is especially adapted for AR coatings which do not incorporate silicon dioxide as the layer occurring just beneath the hydrophobic coating as any AR layer could be used.
In one aspect of the present invention, a coating is applied to a lens which has an AR coating thereon. In other words, the lens has at least two layers on one of the surfaces. The first layer is the AR layer and the second layer is the hydrophobic coating. The AR layer can be of any AR material and may be comprised of several layers of different AR materials. On top of this AR layer, the hydrophobic coating is applied. This hydrophobic coating is preferably a coating which does not significantly affect light transmission and which provides a durable outer layer which is not easily scratched or marred. The hydrophobic coating should also adhere to the underlying AR layer such that it is not easily removed. The hydrophobic coating is preferably an organosilane compound. Preferred organosilane compounds applied to lenses will retain at least 95% of its hydrophobic characteristics after undergoing a standard eraser test. Still more preferably, the hydrophobic coating will retain at least 97% (and even more preferably at least 98.5%) of its hydrophobic characteristics after undergoing a standard eraser test. Most preferably, the hydrophobic coating will retain at least 99.5% of its hydrophobic characteristics after undergoing a standard eraser test. One method of testing the hydrophobic characteristics would include measuring the contact angle of a water drop both before and after the eraser test. Furthermore, when undergoing a standard adhesive tape test, the coating should retain less than 5% of the applied stain with the remaining 95% being removed by the adhesive tape. Still more preferably, the hydrophobic coating will permit removal of at least 97% (and even more preferably at least 98.5%) of the stain undergoing a standard adhesive tape test. Most preferably, the hydrophobic coating will permit removal of at least 99.5% of the applied stain after undergoing a standard adhesive tape test. With respect to light transmission, the hydrophobic coating should have little to no effect and riflescopes containing a plurality of lenses therein should exhibit less than a 5% loss of light through the lenses. Preferably, any one lens having an AR coating on each side of the lens and a hydrophobic coating applied to at least one of the sides of the lens should have at least 99% light transmission therethrough. A particularly preferred organosilane compound is PERMASEAL (Nanofilms, Inc., Valley View, Ohio).
Therefore the principle objects of the present invention are: to provide a telescope lens having an external face that is treated with a coating to resist accumulation of water on the lens to reduce the likelihood of fogging and to reduce the likelihood of distortion due to collection of moisture acting under surface tension thereon; to provide such a lens having a high percentage of transmission of light in the visible range; to provide such a lens wherein the coating comprises a anti-reflective layer and a hydrophobic polymer layer on the surface of the lens; to provide such a lens wherein the anti-reflective layer has a sublayer of silicon dioxide that is adjacent to the hydrophobic layer; to provide such a lens wherein the anti-reflective layer is heated during manufacturing and while on the lens to a temperature above ambient temperature and preferably to a temperature in the range of 250-300xc2x0 centigrade; to provide such a lens that has very close to 100% transmission of light over a very wide spectrum of visible light; to provide a lens having an AR layer and a hydrophobic coating on top of the AR layer wherein the hydrophobic coating provides increased durability while having little to no effect on light transmission; and to provide such a lens which is relatively easy to construct, inexpensive to produce and especially well suited for the intended usage thereof.
Other objects and advantages of this invention will become apparent from the following description taken in conjunction with the accompanying drawings wherein are set forth, by way of illustration and example, certain embodiments of this invention.
The drawings constitute a part of this specification and include exemplary embodiments of the present invention and illustrate various objects and features thereof.